[MemProf] XFAIL test on avr until issue can be debugged

For unknown reasons, this test started failing only on the
llvm-avr-linux bot after 5c20d7d:
http://lab.llvm.org:8011/#/builders/112/builds/365

The error message is not helpful, and I have an email out to the bot
owner to help with debugging. XFAIL it on avr for now.

Author: teresajohnson

compiler-rt/test/memprof/TestCases/log_path_test.cpp

@@ -1,6 +1,10 @@
 // The for loop in the backticks below requires bash.
 // REQUIRES: shell
 //
+// TODO: It isn't clear why this test started failing on the avr bot.
+//       Waiting for help from bot owner to investigate.
+// XFAIL: avr
+//
 // RUN: %clangxx_memprof  %s -o %t
 
 // Regular run.